VPG

1. Introduction

Volume Phase Gratings (VPG) work much like conventional surface relief reflection gratings, except in transmission.  They are periodic phase structures, whose fundamental purpose is to diffract different wavelengths of light from a common input path into different angular output paths.

A VPG is formed in a layer of transmissive material, usually dichromatic gelatin, which is sealed between two layers of clear glass or fused silica.  The phase of incident light is modulated as it passes through the optically thick film that has a periodic differential hardness or refractive index.  Hence the term “volume phase”.  This is in contrast to a conventional grating, in which the depth of a surface relief pattern modulates the phase of the incident light.  This key distinction is shown schematically in Figure 1.

VPGfig1

2. Volume Phase Gratings

2.1 Free-space diffraction gratings

A diffraction grating is a conventional optical device used to spatially separate the different wavelengths or colors contained in a beam of light. The device consists of a collection of diffracting elements (narrow parallel slits or grooves) separated by a distance comparable to the wavelength of light under study. These diffracting elements can be either reflective or transmitting, forming reflection grating or transmission grating. An electromagnetic wave containing a plurality of wavelengths incident on a grating will, upon diffraction, have its electric field amplitude, or phase, or both, modified and, as a result, a diffracting pattern is formed in space. Diffraction gratings can also be classified into two types of gratings: amplitude and phase according to the physical nature of diffracting elements. The former, amplitude grating, is commonly encountered in the textbooks, which is produced through mechanically ruling a thin metallic layer deposited on a glass substrate or photography (lithography) whereas the latter, phase grating, consists of a periodic variation of the refractive index of the grating material. The gratings are known as free-space because the phase difference among diffracted beams is generated in the free space, rather than in dispersion media like waveguides.

2.2 Volume phase gratings

A volume phase grating is also called a thick phase grating according to the well-known Q-parameter, defined as

VPGeq1

where l is wavelength, d is the thickness of the grating, ng is the refractive index of the material, L is the grating period, and a is the incident angle. The phase grating is called  “thin” for Q < 1 and “thick” for Q > 10. The parameters involved are defined in Figure 2.

VPGfig2

BaySpec’s volume phase grating is a thick transmission phase grating, which is designed and manufactured to provide the highest diffracting efficiency (up to 99%) and largest angular dispersion for DWDM devices. The volume phase grating is made from a diffractive element sandwiched between two substrates, each of which is formed from low scattering glass whose external surface is coated with an anti-reflection coating to enhance the passage of radiation. The diffractive element is a volume hologram comprising a photosensitive medium with thickness ranging from a few to tens of micrometers, such as a layer of proprietary photo-polymer materials. Through exposing an interference pattern coming from two mutually coherent laser beams to the photosensitive medium layer, a periodic modulation to the refractive index of the medium is formed, which typically has a sinusoidal profile. This is the volume phase grating. The fabrication of holographic elements for different purposes has been described in several references.  The manufacturing cost of forming holographic elements is low because the work is basically a photographic process.

2.3 Diffraction by volume phase gratings

The high diffraction efficiency and large angular dispersion capability of a volume phase grating provides a proven technology to demultiplex equally spaced DWDM signals. For a thick grating, the diffraction must simultaneously satisfy the well-known grating equation and Bragg condition. From the former, the angular dispersion is found to be

VPGeq2

where m is the diffraction order, L is the grating constant and q is the diffraction angle. On the receiving plane, each channel covers a spatial range determined by

VPGeq3

where L is the distance between the grating and the receiving plane, Dlk is the wavelength range of the k-th channel centered at the wavelength lk. Thus, the outputted channel signals can be obtained at the successive positions x1 = x0+Dx1, x2 = x1+Dx2, x3 = x2+Dx3, …, where xis the reference position. The geometry of a demultiplexer is illustrated in Figure 3.

VPGfig3

BaySpec’s patented VPG® technologies are widely used in most demanding applications in the world.  These include optical channel performance monitors, mux/demux modules, and as a stabilization mechanism for transmission lasers in telecommunications networks, FBG interrogation analyzers for fiber sensing networks, spectrographs for spectral domain optical coherence tomography, and general purpose UV-VIS-NIR and Raman spectroscopy.

VPGfig4

References:

[1]   E.G. Loewen and E. Popov, Diffraction gratings and applications, Marcel Dekker (New York, 1997).

[2]   C. Palmer, Diffraction grating handbook  (Richardson Grating Laboratory, 2001).

[3]   Shu Zhang and Wei Yang, “Compact double-pass wavelength multiplexer-demultiplexer having an increased number of channels”, US Patent No. 6,108,471 (August, 2000).

[4]   Wei Yang and Shu Zhang, “Compact double-pass wavelength multiplexer-demultiplexer”, US Patent No. 6,275,630 (August, 2001).